Molded steel alloy, corresponding part, and manufacturing method

ABSTRACT

This molded steel alloy includes the following elements in weight percent:
         carbon (C) between 0.08% and 0.4%,   silicon (Si) between 0.15% and 2%,   nickel (Ni) between 24% and 31%,   cobalt (Co) between 15% and 30%, and   niobium (Nb) between 0.01% and 2.5%.       

     The composition also includes an additional element selected from a group consisting of: molybdenum (Mo) at a content of less than or equal to 3% by weight, manganese (Mn) at a content of less than or equal to 1.5% by weight, chromium (Cr) at a content of less than or equal to 1.5% by weight, phosphorus (P) at a content of less than or equal to 0.04% by weight, sulfur (S) at a content of less than or equal to 0.03% by weight, copper (Cu) at a content of less than or equal to 0.5% by weight, iron, and unavoidable impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

See Application Data Sheet.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

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INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR ASA TEXT FILE VIA THE OFFICE ELECTRONIC FILING SYSTEM (EFS-WEB)

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STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINTINVENTOR

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BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a steel alloy made in foundry, thecomposition of which includes inter alia iron (Fe), carbon (C), nickel(Ni) and cobalt (Co).

The present invention will find its application mainly in the field ofthe manufacture of tools in foundry from a steel alloy, said tools beingafterwards used for forming various parts, namely obtained, in turn,from an alloy including, for example, titanium.

2. Description of Related Art Including Information Disclosed Under 37CFR 1.97 and 37 CFR 1.98

From European patent document EP 0 343 292 is known a steel alloyincluding the following elements in the indicated proportions: C between0.4 and 0.8%; Si less than or equal to 1%; Mn less than or equal to 1%;Ni between 30 and 40%; Co between 2 and 8%; S less than or equal to0.2%; P less than or equal to 0.2%; Mg and/or Ca less than or equal to0.3%; with Ni+Co*0.75 between 32 and 40%, the remainder being iron andunavoidable impurities. The alloy is heated at a temperature between 600and 1000° C. and is then tempered.

This alloy is of interest in the manufacture of mechanical precisionmachines, for example machine-tools, measuring instruments,semiconductor manufacturing machines and optical machines, due to a lowexpansion coefficient at temperatures between 20 and 100° C.

As regards JP 2003 138336, which describes a steel alloy includingbetween 26 and 32% Ni, between 5 and 12% Co, between 0.5 and 1.7% C,less than 1% Si, less than 0.5% Mn, the remainder being iron, said alloyincluding a proportion of rare earths (lanthanum, cerium, praseodymiumand neodymium) between 0.01 and 0.009% and having a low thermalexpansion coefficient up to a temperature of 200° C.

However, in the field covered by the alloy of the present invention, themanufacture of foundry tools made of a steel alloy, the alloy inquestion must have a low thermal expansion coefficient at temperaturesmuch higher than 100° C., and even than 200° C.

Also known in the state of the art are tools specifically made infoundry for forming parts made of titanium alloy at temperatures rangingfrom 400 to 1000° C. The expansion coefficients of these tools arerelatively high (higher than 17·10⁻⁶ K⁻¹).

Now, novel methods for producing parts require tools with a thermalexpansion coefficient close to that of the alloys to be formed, namelyfor titanium alloys, in the range of 10.3×10⁻⁶ K⁻¹ at 830° C., withinthe temperature range used for the forming.

The tools must also have good creep rupture properties. Indeed, such afeature permits to avoid the tools from deforming at high temperature.

For forming temperatures above 400° C., tools with a low expansioncoefficient exist, manufactured only by machine-welding. However, thelatter are likely to deform at high temperature.

For forming temperatures up to 400° C. are known in the state of the arttools made in foundry from a number of alloys having a thermal expansioncoefficient and mechanical strength, which is not the case for highertemperatures.

Thus is known the patent application FR 3 025 807, also filed by theapplicant, which describes an alloy of spheroidal graphite or lamellargraphite cast iron, known as Ferrynox N29K, and comprising the followingelements: carbon (C) between 1.2% and 3.5%—silicon (Si) between 1.0% or1.2% and 3%—nickel (Ni) between 26% and 31%—cobalt (Co) between 15% and20% and optionally: magnesium (Mg) between 0.02% and 0.10%—manganese(Mn) less than or equal to 1.5%—chromium (Cr) less than or equal to 0.5%and/or phosphorus (P) less than or equal to 0.12 or 0.04% and/or sulfur(S) less than or equal to 0.11 or 0.03%, and/or molybdenum (Mo) lessthan or equal to 0.5%, and/or copper (Cu) less than or equal to 0.5%,the remainder being iron and unavoidable impurities.

This alloy permits to obtain tools having an interesting, i.e. low andstable, thermal expansion coefficient for temperatures up to 400° C. Theso manufactured tools have an interesting application in the manufactureof parts made of composite or thermoplastic materials.

However, the expansion coefficient at temperatures above 400° C. is toohigh.

For temperatures above 400° C., there exist however alloys with a lowthermal expansion coefficient for tools made in foundry, but their creeprupture properties is low.

Now, some materials, such as titanium alloys for example, can only beformed at temperatures above 400° C.

Therefore, the tools used for forming them must have not only a lowthermal expansion coefficient, but also good creep rupture properties,in order to avoid a deformation of said tools at temperatures above 400°C. during the forming, for example, of parts made of titanium alloy.

BRIEF SUMMARY OF THE INVENTION

The aim of the invention is to permit the manufacture by molding of apart made of steel the thermal expansion of which is low at hightemperatures and namely up to 1000° C., and which exhibits good creeprupture properties.

In particular, the aim of the invention is to design an alloy thatpermits to manufacture tools having a low expansion coefficient and goodcreep rupture properties, including at very high temperatures.

To this end, the subject matter of the invention is a steel alloycharacterized in that it is comprised, in % by weight with respect tothe total weight of the alloy, of the following elements:

carbon (C) between 0.08% and 0.4%,

silicon (Si) between 0.15% and 2%,

nickel (Ni) between 24% and 31%,

cobalt (Co) between 15% and 30%,

niobium (Nb) between 0.01% and 2.5%,

the remainder being iron and unavoidable impurities.

Optionally, the alloy according to the invention comprises molybdenum(Mo) at a content of less than or equal to 3% by weight and/or manganese(Mn) at a content of less than or equal to 1.5% by weight and/orchromium (Cr) at a content of less than or equal to 1.5% by weightand/or phosphorus (P) at a content of less than or equal to 0.04% byweight and/or sulfur (S) at a content of less than or equal to 0.03% byweight and/or copper (Cu) at a content of less than or equal to 0.5% byweight.

According to particular embodiments, the alloy according to theinvention may include one or more of the following features:

-   -   the nickel (Ni) content is between minimum 24%, or minimum 25%        or minimum 26% and maximum 30%, or maximum 30.5% or maximum 31%        by weight;    -   the cobalt (Co) content is between minimum 15%, or minimum 16%        and maximum 19%, or maximum 27% or maximum 30% by weight;    -   the niobium (Nb) content is between minimum 0.01%, or minimum        0.05% and maximum 1.5%, or maximum 2%, or maximum 2.5% by        weight;    -   the carbon (C) content is between minimum 0.08%, or minimum        0.1%, or minimum 0.12% and maximum 0.2%, or maximum 0.3% or        maximum 0.4% by weight;    -   the silicon (Si) content is between minimum 0.15%, or minimum        0.2% and maximum 1%, or maximum 1.5%, or maximum 2% by weight;    -   the molybdenum (Mo) content is between minimum trace amounts or        minimum 0.05% or maximum 0.5% and maximum 1.2%, or maximum 1.5%,        or maximum 3% by weight;    -   the manganese (Mn) content is between minimum trace amounts or        minimum 0.05%, or minimum 0.1% and maximum 1%, or maximum 1.5%;    -   the copper (Cu) content is less than or equal to 0.5% by weight,        preferably less than or equal to 0.4%, preferably less than or        equal to 0.3% by weight.    -   the chromium (Cr) content is less than or equal to 1.5% by        weight, preferably less than or equal to 1% by weight,        preferably less than or equal to 0.4% by weight, preferably less        than or equal to 0.3% by weight.

The subject matter of the invention is also a part made at least in partfrom a steel alloy, said alloy having a composition as defined above.

This part may namely consist of a tool, the purpose of which is topermit the forming of other parts made of alloys, for exampletitanium-based alloys. In other words, said tool made of the alloyaccording to the invention is likely to receive an alloy at a hightemperature between 400 and 1000° C., and it must have a low expansioncoefficient and good creep rupture properties within this temperaturerange, and namely up to a forming temperature of 1000° C.

The invention also relates to a method for manufacturing a part asdefined above, characterized in that it includes at least the followingsteps:

preparing an alloy in foundry that has a composition as defined above;

said alloy is poured into a mold in order to obtain said part.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The FIGURE is a graph illustration, showing the thermal expansionbehavior of four alloys.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be better understood when reading the followingdetailed description, given only by way of an example and with referenceto the single FIGURE in which is shown the thermal expansion behavior offour alloys (GX40CrNiSi25-20, GXX40NiCrSi38-19, Ferrynox N29K andExample 1), among which an example of an alloy according to theinvention (Example 1), by means of the evolution of their averagethermal expansion coefficient (in 10⁻⁶ m/m° C. or 10⁻⁶ K⁻¹) depending onthe temperature (in ° C.) and also with reference to the table below,which gives the creep rupture properties of these different alloys.

The subject matter of the invention is a particular alloy composition.This alloy permits to obtain parts the thermal expansion coefficient ofwhich is low and the creep rupture properties of which are high up to atemperature of 1000° C.

The part is for example a tool, namely a tool for forming metal parts.

All the compositional indications are given below in % by weight of thetotal weight of the alloy.

A first aspect of the invention is the chemical composition of thealloy.

The alloy is a molded steel alloy made in foundry. Its basic componentis iron (Fe). It also includes unavoidable impurities resulting from thepreparation of said alloy.

The alloy according to the invention has a composition comprised,besides iron (Fe) and said impurities, of carbon (C) between 0.08% and0.4%, silicon (Si) between 0.15% and 2%, nickel (Ni) between 24% and31%, cobalt (Co) between 15% and 30% and niobium (Nb) between 0.01% and2.5%.

The nickel (Ni) content of the alloy may preferably vary between atleast 25% or 26% and at most 30% or 30.5% by weight.

The cobalt (Co) content of the alloy may preferably vary between atleast 16% and at most 19% or 27% by weight.

The niobium (Nb) content of the alloy may preferably vary between atleast 0.05% and at most 1.5% or 2% by weight.

The carbon (C) content of the alloy may preferably vary between at least0.1% or 0.12% and at most 0.2% or 0.3% by weight.

The silicon (Si) content of the alloy may preferably vary between atleast 0.2% and at most 1% or 1.5% by weight.

In addition, the alloy may comprise manganese (Mn) at a content betweentrace amounts and 1.5%.

The manganese (Mn) content of the alloy may preferably vary between atleast 0.05% or at least 0.1% and at most 1.0% by weight.

In addition, the alloy may comprise copper (Cu) at a content betweentrace amounts and 0.5% by weight.

The copper (Cu) content of the alloy may preferably be less than 0.4%,yet more preferably less than 0.3% by weight.

In addition, the alloy may comprise molybdenum (Mo) at a content betweentrace amounts and 3%.

The molybdenum (Mo) content of the alloy may preferably vary between0.05% or 0.5% and at most 1.2% or 1.5% by weight.

In addition, the alloy may comprise chromium (Cr) at a content betweentrace amounts and 1.5%.

The chromium (Cr) content of the alloy may preferably be less than orequal to 1% by weight, more preferably less than or equal to 0.4% and,most preferably, this chromium content is less than or equal to 0, 3%.

In addition, the alloy may comprise phosphorus (P) at a content betweentrace amounts and 0.04% by weight.

In addition, the alloy may comprise sulfur (S) at a content betweentrace amounts and 0.03% by weight.

Since the end of the 19th century has been known an iron-based alloywith a low expansion coefficient, which is Fe—Ni36. This alloy is knownunder the trade name INVAR® (TRADEMARK).

In its composition, this alloy includes 36% by weight of nickel, theremainder being iron.

The curve representing the expansion coefficient of the iron/nickelalloys shows an anomaly around 36% nickel: the expansion coefficient isthen much lower than for the other compositions. However, this onlyapplies for low temperatures up to 130° C., above which the expansioncoefficient of the alloy is no longer stable. Thus, the implementationof this alloy at higher temperatures is not advantageous.

Various alloys were then developed starting from this base, namely withcobalt as an addition element. For example, iron/nickel/cobalt steelwith 32% nickel and 5.5% cobalt has a lower expansion coefficient thanINVAR®, and especially retains this property at higher temperatures.However, its creep rupture properties at high temperature are low.

Known are molded refractory steels (GX40CrNiSi25-20, GX40NiCrSi38-19),which now permit to obtain good creep rupture properties at hightemperature (rupture stress in 1000 h equal to 28 and 30Mpa,respectively, at 900° C.), but with a high expansion coefficient (18.6and 17.3·10⁻⁶ K⁻¹, respectively, at 900° C.).

According to examples, the alloy comprises, besides iron (Fe) and theunavoidable impurities, only the following elements, within theindicated limits:

C Si Mn P S Cr Ni Mo Cu Co Nb Mini 0.08 0.15 traces traces Traces traces24 Traces traces 15 0.01 Maxi 0.4 2 1.5 0.04 0.03 1.5 31 3 0.5 30 2.5Example 0.14 1.49 0.14 0.014 0.008 0.12 28.22 0.0051 0.038 16.7 0.59 1Example 0.13 1.1 0.4 0.015 0.007 0.11 27.79 0.01 0.045 25.1 0.64 2

This alloy is a molded steel.

The thermal expansion coefficient of Example 1, as compared with thermalexpansion coefficients of the three alloys known from the prior art(GX40CrNiSi25-20, GX40NiCrSi38-19, Ferrynox N29K) is shown in the singleFIGURE.

The thermal expansion coefficient of the alloy having the composition ofExample 1 is less than 12.6*10⁻⁶ K¹ at a temperature below 980° C.

More generally speaking, one observes that the thermal expansioncoefficient of the alloy having the composition of Example 1 issignificantly lower than the expansion coefficient of the standardizedalloys GX40CrNiSi25-20 and GX40NiCrSi38-19, and in nearly the entiretemperature range that has been tested, in particular for hightemperatures.

Therefore, the alloy according to the invention has a low expansioncoefficient at temperatures between 400 and 1000° C., applied during theforming of parts made of titanium alloy, which is of particularinteresting.

The creep rupture properties of the alloy according to the invention andhaving a composition according to Example 1 was also tested and comparedwith those of the alloys GX40CrNiSi25-20 and GX40NiCrCrSi38-19 and thealloy known as Ferrynox N29K.

More specifically, and as a reminder, the Ferrynox N29K consists of aspheroidal graphite, or lamellar graphite, cast-iron alloy, with lowexpansion and subject matter of patent application FR 3 025 807.

The creep rupture properties are as follows:

Example 1 Ferrynox N29K GX40CrNiSi25-20 GX40NiCrSi38-19 Breaking stressBreaking stress Breaking stress Breaking stress (MPa) (MPa) (MPa) (MPa)100 h 1000 h 100 h 1000 h 100 h 1000 h 100 h 1000 h at 700° C. 85 68 7052 100 80 80 at 800° C. 62 46 49 33 75 50 90 50 at 900° C. 39 25 21 1447 28 48 30 at 980° C. 21 14 12 8 28 16 28 17

The creep rupture properties of the steel alloy according to theinvention having the contents according to Example 1 are much higherthan those of the alloy known as Ferrynox N29K.

These improved creep rupture properties are observed for the alloy ofExample 1 at all the temperatures that have been tested and that arelikely to be used during the manufacture of parts made of titanium, andthis as well after 100 hours or after 1000 hours of exposure.

The improvement of the creep rupture properties is due to the presence,in the composition of the alloy according to the invention, of niobium,which causes the formation at the grain boundaries of niobium carbides.Niobium carbides permit a blocking of creep by preventing the grainsfrom sliding relative to each other.

Furthermore, the breaking stress value at 1000 h of a part obtained fromthe alloy of Example 1, namely 25 MPa at 900° C., is comparable to thebreaking stresses of the alloys GX40CrNiSi25-20 and GX40NiCrSi38-19, 28and 30 MPa, respectively, at 900° C., as indicated above. Now, thesealloys GX40CrNiSi25-20 and GX40NiCrSi38-19 are considered to have goodcreep rupture properties at high temperature.

Another aspect of the alloy according to the invention is that it isweldable.

A second aspect of the invention is a part made of an alloy as definedabove. The part is naming a tool. The tool may comprise only portionsmade of the alloy according to the invention or may entirely be made ofthis alloy.

A third aspect of the invention is the method for manufacturing a partmade of an alloy according to the invention.

First of all, the alloy is prepared in foundry with the compositionsthat have been described above. In other words, the various constituentsare mixed according to the contents that have been indicated.

Said alloy is then poured into a mold having a shape and dimensionsconfigured to permit the manufacture of the desired part, the partconsisting, for example, of a tool.

After the cooling step, the part can be heat-treated.

The alloy is used namely for manufacturing tools implemented thereafterfor forming metal parts (for example made of titanium alloy).

The technical field can be aeronautics.

1. A steel alloy composition, comprising in weight percent: carbon (C)between 0.08% and 0.4%, silicon (Si) between 0.15% and 2%, nickel (Ni)between 24% and 31%, cobalt (Co) between 15% and 30%, niobium (Nb)between 0.01% and 2.5%, and an additional element selected from a groupconsisting of: molybdenum (Mo) at a content of less than or equal to 3%by weight, manganese (Mn) at a content of less than or equal to 1.5% byweight. chromium (Cr) at a content of less than or equal to 1.5% byweight, phosphorus (P) at a content of less than or equal to 0.04% byweight, sulfur (S) at a content of less than or equal to 0.03% byweight, copper (Cu) at a content of less than or equal to 0.5% byweight, iron, and impurities.
 2. The steel alloy according to claim 1,wherein the nickel (Ni) content is minimum 25% or minimum 26% andmaximum 30% or maximum 30.5% by weight.
 3. The steel alloy according toclaim 1, wherein the cobalt (Co) content is minimum 16% and maximum 19%or maximum 27% by weight.
 4. The steel alloy according to claim 1,wherein the niobium content (Nb) is minimum 0.05% and maximum 1.5% ormaximum 2% by weight.
 5. The steel alloy according to claim 1, whereinthe carbon content (C) is minimum 0.1% or minimum 0.12% by weight andmaximum 0.2% or maximum 0.3% by weight.
 6. The steel alloy according toclaim 1, wherein the silicon content (Si) is minimum 0.2% and maximum 1%or maximum 1.5% by weight.
 7. The steel alloy according to claim 1,wherein the molybdenum content (Mo) is minimum 0.05% or minimum 0.5% andmaximum 1.2% or maximum 1.5% by weight.
 8. The steel alloy according toclaim 1, wherein the manganese content (Mn) is minimum 0.05% or minimum0.1% and maximum 1% by weight.
 9. The steel alloy according to claim 1,wherein the content of copper (Cu) is less than or equal to 0.4% byweight, preferably less than or equal to 0.3% by weight.
 10. The steelalloy according to claim 1, wherein the chromium (Cr) content is lessthan or equal to 1%, preferably less than or equal to 0.4%, preferablyless than or equal to 0.3% by weight.
 11. A part, comprising: a steelalloy being comprised of a composition according to claim
 1. 12. Amethod for manufacturing a part, the method comprising the followingsteps: preparing an alloy in a foundry, having a composition accordingto claim 1; pouring said alloy into a mold in order to obtain a part.